If it's spare HOV capacity, then I agree. But if adding the electric cars to HOV lanes increases the congestion in HOV lanes, it might degrade their effectivene

If it's spare HOV capacity, then I agree. But if adding the electric cars to HOV lanes increases the congestion in HOV lanes, it might degrade their effectiveness in incentivizing the behaviors they currently incentivize. For example, the attractiveness of either carpooling or riding the bus declines if HOV lanes get slower. Carpooling in particular needs enough of a delta in the regular v. HOV trip times to make up for having to pick up the other carpool members (unless you live in the same place). One solution is to allow a capped number in, to incentivize purchase but only up to a threshold that doesn't overly degrade HOV trip times. I believe that's what California did for hybrids, allowing HOV usage for the first N purchased.